- Will automatically change icon for filetype component when vim's
  filetype option changes

- Idiomatically derive icon for the filetype component from buffer's
  filetype (instead of deriving from filename first).

- Some icons may change. E.g. markdown files from "" to "", because
  nvim-web-devicons have different icons for "markdown" (which is
  derived from filetype) and "md" (which is derived from file
  extension).